In recent years, extensive studies have been made to utilize the sunlight which is unlimited and generates no toxic substances. Examples of the practical application of sunlight which is a clean energy source, at present, include inorganic solar cells for a residential application such as single crystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride and indium-copper selenide.
However, one of the disadvantages of these inorganic solar cells is that, for example, in the case of a silicon solar cell, very high purity silicon is required, in which, naturally, the purification process is complicated and numerous processes are included, resulting in a high manufacturing cost.
On the other hand, many solar cells utilizing an organic material have also been proposed. Examples of an organic solar cell include: (i) a Schottky-type photoelectric conversion element in which a p-type organic semiconductor and a metal having a small work function are connected; and (ii) a hetero-connection type photoelectric conversion element, in which a p-type organic semiconductor and an n-type inorganic semiconductor are connected or a p-type organic semiconductor and an electron accepting organic compound are connected. Organic semiconductors utilized in such an organic solar cell include, for example, a synthetic dye or a pigment such as chlorophyll and perylene; and a conductive polymer material such as polyacetylene, and complex materials thereof. These materials are made into thin film, by such as a vacuum evaporation method, a casting method or a dipping method, which constitutes a battery material. The organic material has advantages, for example, a low cost and easy application to a larger area; however, there are also problems, for example, conversion efficiency as low as not more than 1% in many materials; and poor durability.
In such a situation, a solar cell exhibiting excellent characteristics has been reported by Dr. Gratzel et al., Switzerland (for example, refer to Non-Patent Document 1). The proposed cell is a compound sensitized solar cell, and is a wet type solar cell utilizing titanium oxide porous thin film, which is spectrally sensitized by a ruthenium complex, as a working electrode. Advantages of this method are that (i) a low priced oxide semiconductor such as titanium oxide can be used and the purification up to a high purity of this material is not required, resulting in attaining a low cost; and that (ii) usable light covers a broad visible light region, which enables efficient conversion of sunlight to electricity, since sunlight is rich in a visible light component.
On the contrary, since a ruthenium complex having a resource limitation is utilized, supply of a ruthenium complex is uncertain when this solar cell is utilized in practice. Further, this ruthenium complex is expensive and has a problem of stability in aging, however, this problem will be overcome if the material can be changed into a low priced and stable organic compound.
It has been disclosed that an element having high photoelectric conversion efficiency can be obtained when a compound having a triphenylamine structure is utilized (for example, refer to Patent Document 1). However, it has been found that these compounds have problems, for example, a relatively low adsorptive property to titanium oxide, not very high sensitizing effect and not fully sufficient durability.
Patent Document 1 Japanese Patent Application Publication Open to Public Inspection No. (hereinafter, referred to as JP-A No.) 2005-123033
Non-Patent Document 1 B. O'Regan and M. Gratzel, Nature, 353, 737